Time to attack. Dip your bottle brush in the vinegar solution. Scrub the inside walls of the reservoir with enthusiasm. Pretend you’re washing a very weird, very wet dinosaur egg.
Use a small toothbrush for the tiny crevices. Focus on the water intake valve and the float ball. If it moves, scrub it. If it doesn’t move, scrub it anyway.
Don’t forget the drip tray. Pop it off and wash it with soap and hot water. You’ll find sand-like grit from sediment. That’s your tap water saying hello. Rinse well.
